The technology of encoding information on various articles with bar codes is well known. Traditional bar code systems rely on the reflection differences of the reading light from the black (light-absorbing) bars and the white (light-reflecting) spaces of the bar code. A typical laser bar code reader scans the beam from a helium-neon or diode laser across the bar code. Photo-detectors monitor the beam's reflectance from the bars and spaces, and the resultant electronic signals are processed and decoded.
Bar codes on mail can be used in a system for automatic sorting of mail in the postal system. For this application, ordinary bar codes would be difficult to read over dark backgrounds and could obscure underlying printed material. Luminescent bar codes may include both fluorescent and phosphorescent materials that can be formulated into relatively clear luminescent inks that do not obscure the underlying printed material on mail pieces. In many cases background obscuration is less important than the ability to read bar codes printed over backgrounds of widely varying reflectivity. In these cases luminescent bar codes may be printed using luminescent ink formulations that are more opaque.
Bar code readers require sufficient contrast between bars and spaces to permit accurate differentiation. With fluorescent bars, a major problem has been the background fluorescence of the paper in the spaces of the bar code. Various ways of compensating for differences in background fluorescence of the spaces have been devised by use of optical filters and electronic circuitry (U.S. Pat. No. 3,207,910 Hirschfeld) and development of fluorescent inks that can be activated by ultraviolet light to fluoresce at longer wavelengths (e.g. 580 nm peak) where the background fluorescence of the paper is less (U.S. Pat. No. 4,186,020, Wachtel). Methods are known to deal with the background reflectance of the paper on which a luminescent bar code is printed, which may appreciably affect the luminescent signal generated by the bar code due to the background under the bars of the bar code. This latter system is disclosed in U.S. Pat. No. 4,983,817 to Dolash et al, which is incorporated herein by reference as fully as if it were presented in complete text. In U.S. Pat. No. 5,380,992 to Damen et al there is disclosed a method and apparatus for compensating for background luminescence so that background luminescence does not affect bar code/no bar code detection decisions. The Dolash patent however does not solve a problem created by luminescent inks that, when used for bar code printing over a given background, the luminescent inks have a reflectivity at the source excitation wavelength that is measurably less than the reflectivity of the background material itself at source excitation wavelengths. In cases where for a given background, there is a difference in reflectivity of the bars printed with luminescent ink, and the reflectivity of the space between printed bars, neither Dolash et al nor Damen et al can be used to automatically compensate the luminescent bar code signals without distorting the signal at bar code edge transitions.
It is well known that material properties other than reflectivity may affect the signal levels received by the luminescent channel and reflectance channel detectors. Porous materials tend to absorb inks, while non-porous materials permit inks to puddle or stand up more on the surface. Background luminescence inherent in the substrate and inks used on the substrate will likewise affect signal levels. In applications where it is necessary to compensate for bar codes printed over a wide range of such different materials, the bar code compensation apparatus and method disclosed and claimed herein will account for this.
For successful decoding with luminescent bar codes, it is necessary that the processed signal from the code delivered to the decoder have practically constant amplitude, independent of any variations in the collected luminescent signal due to localized variations in background reflectance. Thus, means are needed for automatically compensating the total collected luminescent signal for the reflected component of the luminescent light according to the localized background and bar code surface reflectance in the area of the scanning beam. The present invention provides such means.
It is well known to use optical filters to separate light of difference frequencies. For example, in traditional bar code readers, a filter is used to block the reflected ambient light and pass only the reflected light from the bar codes. To adapt this traditional system to reading infrared luminescence, the conventional ambient-light filter is replaced by an optical filter that blocks the light of the laser and passes the luminescent light in the far-red and near-infrared range of the spectrum. The amount of excitation light to be blocked by the filter depends on the amount reflected by the background on which the luminescent bar code is printed. It is greatest for a reflective white surface and least for a dark light-absorbent surface such as black printing ink.
If, in addition to blocking the reflected excitation light in order to selectively measure luminescent light, separate means are provided to simultaneously measure the amount of reflected excitation light, a separate electrical or optical signal can be obtained that is related to the reflectance at excitation wavelengths of the bar code and the substrate over which it was printed. This signal can be used to simultaneously compensate the amplitude of the luminescent signal by appropriate electronic or optical circuitry to provide a relatively constant amplitude of the signal fed to the decoder.
The compensation method described in the Dolash Patent significantly improved the ability to detect transparent and somewhat transparent bar codes printed with luminescent materials. This type of compensation can, however, create a distorted rendition of the signal representing the bar code edges. The resulting distortion can ultimately lead to the misinterpretation of bar code widths, or in certain cases, bar codes can go undetected. There are several contributors to this potential distortion. In the case of somewhat transparent bar codes, or bar codes that are more opaque at excitation wavelengths, the reflected signal over bars is generally less than it is over spaces between bars for a given background. Thus, during the transition from bar to space, both the reflected signal amplitude and the luminescent signal amplitude change. In this case, the compensation of the Dolash patent does not produce a signal that represents a true rendition of the transition from bar to space or from space to bar.
Another source of performance degradation results when optical filters used to separate the two channels do not completely prevent reflected signal leakage in the luminescent channel (crosstalk). Since the Dolash patent relies on the luminescent signal to be near zero in bar code spaces regardless of the reflected signal, it is important that the leakage be minimal in order for the compensation scheme to maintain the widest possible dynamic range. The present invention is more tolerant of channel crosstalk, because the calibration procedure includes a measure of the phenomenon, and the compensation algorithm takes crosstalk into account.
It is common for bar codes to be printed using ink jet printers. Printed bars are formed by a series of ink dots, where the diameter of the ink dots is approximately equal to the spacing between the ink dots. For scanning bar code systems, maximum power is detected by the receiver when the receiver aperture is equal to or greater than the laser spot size. It is common practice to adjust the laser spot size equal to seven tenths of the width of the narrowest bar or space to be detected at the nominal read distance. This permits maximum signal modulation, and still compensates for some voids in the bar. Thus when the bar code scanning system is set up properly, the electrical or optical signal from the narrowest bar is in a constant state of transition. Bar code signal pulse widths are normally measured at the 50% point on the transitions. In order to maintain optimum noise margins, it is preferable that linear bar to space and space to bar transitions be maintained. The present invention will assure that the best rendering of the actual bar code that was printed will be obtained so that the binarized signal output is an accurate reproduction of the width of the bars.
The apparatus and the calibration method described in the present invention can maintain bar code edge signal transition linearity, or optionally select a nonlinear transfer function for correcting the bar code edge signal. A linear case is described in detail herein. A non linear transfer function for the bar code edge signal would be preferable in a case where bar detection is more important than being able to measure the width of the bar or bar space.